Previous studies have shown that the cell surface expression of thymus leukemia antigen (TL) on ASL-1w leukemia cells varies with the progression of the cells through the growth cycle. Expression of TL is maximal in S phase and its quantitative expression varies directly with DNA synthesis. In the present study the effect of anti-TL serum on the growth of ASL-1w cells was examined. The antiserum, tested in the absence of complement, affected the growth of these cells in biphasic manner. When the antiserum concentration was 0.1% or greater, there was a rapid decline in the rate of DNA synthesis and, after 5-7 hours, cell death occurred. When the antiserum concentration was 0.067% or less, the decline in the rate of synthesis of DNA did not become apparent until 5-6 hours after treatment. Under these conditions there was approximately a 20% increase in cell number after 24 hours of culture. The hypothesis that treatment of ASL-1w cells with the lesser concentration of anti-TL serum blocks the cells in G-2 phase of the cell cycle is discussed. The objective of this proposal is to test this hypothesis. Relevant experiments which will be carried out are: 1) Biosynthesis of TL and relationship of cell cycle to regulation. 2) Use of various cell cycle specific drugs to determine their effect on the regulation of biosynthesis of TL. 3) Use of synchronous cell cultures to study the relationship between TL and regulation of cell division.